1. Field of the Invention
The present invention relates to an exhaust emission control device and method for an internal combustion engine, and an engine control unit, and more particularly to an exhaust emission control device and method for an internal combustion engine including a NOx catalyst that is disposed in the exhaust system of the engine, for trapping NOx in exhaust gases emitted from the engine, and an engine control unit.
2. Description of the Related Art
Conventionally, there has been proposed an exhaust emission control device of this kind, such as an exhaust emission control device for a diesel engine (hereinafter simply referred to as “the engine”), e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2003-166415. In this exhaust emission control device, to restore the NOx trapping performance of a NOx catalyst for regeneration of the NOx catalyst, a regeneration operation in which SOx deposited on the NOx catalyst is reduced and the reduced SOx is released is carried out as follows: Post injection for injecting fuel from a fuel injection valve during the expansion stroke or the exhaust stroke is carried out, and the amount of fuel injected by the post injection and the amount of EGR gases are controlled, whereby the temperature of the NOx catalyst is held not lower than a predetermined temperature that is suitable for reduction of SOx. In this state, by controlling a reducing agent addition valve provided in an exhaust pipe, supply of SOx-reducing fuel to the NOx catalyst and interruption thereof are repeatedly carried out, whereby the NOx catalyst is prevented from being overheated by combustion of fuel supplied from the reducing agent addition valve.
As described above, in the conventional exhaust emission control device, the supply of SOx-reducing fuel is interrupted, and hence to cause the NOx catalyst to completely release SOx deposited thereon, it is required to set a long execution time period of the regeneration operation. In the conventional exhaust emission control device, however, to control the temperature of the NOx catalyst to a predetermined value, the post injection that does not contribute to an output of the engine is carried out during execution of the regeneration operation, so that when the execution time period of the regeneration operation is set to be long as described above, the amount of fuel injected by the post injection increases to degrade fuel economy. Further, since supply of fuel by the reducing agent addition valve, and control of the amount of EGR gases for controlling the temperature of the NOx catalyst are executed in parallel independently of each other, the amount of EGR gases is sometimes controlled such that it becomes too small or too large, irrespective of the load on the engine. In such a case, exhaust emissions increase, and drivability is degraded due to the incapability of obtaining a desired output of the engine.